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| Titel |
Air-snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS |
| VerfasserIn |
K. Toyota, A. P. Dastoor, R. M. Staebler, J. C. McConnell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070262
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| Zusammenfassung |
| A dynamic exchange of halogens between the ocean, sea ice, snowpack, and the
atmosphere is a main driver for the occurrence of ozone depletion episodes (ODEs) and
atmospheric mercury depletion episodes (AMDEs) in the polar boundary layer particularly
during the spring. Oxidized mercury is deposited to the snow/ice surface efficiently
concurrent with the AMDEs and can be transformed to methyl-mercury, which
subsequently bio-magnifies and imposes various health threats to northern communities
and wild life. However, some field measurements of mercury in the snowpack and
overlying ambient air, including but not limited to those in the polar region, indicate the
photochemical reduction of oxidized mercury back to gaseous elemental mercury (GEM)
on timescales of days to weeks whereas other studies show no evidence of rapid
reduction. Such differences could be attributed not only to meteorological factors like
temperature but also to chemical/biological factors that control the abundance of halogens
and organic compounds, with a link to the redox chemistry of mercury. In order to
understand the role of each driving process in the overall behaviors of mercury in the
polar region, we have developed a one-dimensional model, PHANTAS (a model of
PHotochemistry ANd Transport in Air and Snowpack), which describes multiphase
chemistry in the gas phase, aerosols and the brine layer assumed to exist on the grain
surface of saline snowpack. Henry’s law for Hg(II) gases and aqueous-phase stability
constants for Hg(II)-halide complexes are re-evaluated including their temperature
dependence. Photochemical reduction of Hg(II) to Hg(0) in the aqueous phase is
handled simply by a prescribed first-order rate constant with diurnal variations. The
model also handles the transport of gases and aerosols across the snowpack and
the turbulent atmospheric boundary layer. The atmospheric profile of turbulent
diffusivity down to the interfacial sublayer is diagnosed from an arbitrary chosen set of
measured surface sensible heat fluxes, reference-height wind speed and static stability
in the free troposphere. The model yields a shallower boundary layer depth with
decreasing wind speed, leading to more rapid ODEs and AMDEs. On the other
hand, the amount of Hg(II) deposition is simulated to increase with increasing wind
speed. Ozone and GEM are actively destroyed in the snowpack interstitial air via
bromine radical chemistry. However, apparent dry deposition velocities for ozone (and
GEM where efficient Hg(II) reduction is not included in the model) reached only
up to the order of 10-3 cm/s. The gas-particle partitioning of oxidized mercury
in the air is strongly connected to bromine chemistry in that particulate mercury
starts to build up mainly as HgBr42- in sulfate aerosols after ozone is significantly
depleted. In the saline snowpack above the sea ice, mixed-halide complexes like
HgCl3Br2- and HgCl2Br22-, as well as HgCl42-, are simulated to comprise a
major component of inorganic Hg(II). A predominant fraction of Hg(II) entering
from the atmosphere is captured in the top millimeter of the snowpack, whereas
molecular diffusion in the brine and re-emission of GEM followed by re-oxidation in
the interstitial air contribute to the downward migration of some of the Hg(II). |
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